1. Field of the Invention
The invention relates to superhard polycrystalline material elements for wear, cutting, drawing, and other applications where engineered superhard surfaces are needed. The invention particularly relates to polycrystalline diamond compacts (collectively called PDC) cutting elements with greatly improved wear resistance and methods of manufacturing them.
2. Description of Related Art
Polycrystalline diamond and polycrystalline diamond-like elements are known, for the purposes of this specification, as PDC elements. PDC elements are formed from carbon based materials with exceptionally short inter-atomic distances between neighboring atoms. One type of polycrystalline diamond-like material is known as carbonitride (CN) described in U.S. Pat. No. 5,776,615. Another, more commonly used form of PDC is described in more detail below. In general, PDC elements are formed from a mix of materials processed under high-temperature and high-pressure into a polycrystalline matrix of inter-bonded superhard carbon based crystals. A common trait of PDC elements is the use of catalyzing materials during their formation, the residue from which, often imposes a limit upon the maximum useful operating temperature of the element while in service.
A well known, manufactured form of PDC element is a two-layer or multi-layer PDC element where a facing table of polycrystalline diamond is integrally bonded to a substrate of less hard material, such as tungsten carbide. The PDC element may be in the form of a circular or part-circular tablet, or may be formed into other shapes, suitable for applications such as hollow dies, heat sinks, friction bearings, valve surfaces, indentors, tool mandrels, etc. PDC elements of this type may be used in almost any application where a hard wear and erosion resistant material is required. The substrate of the PDC element may be brazed to a carrier, often also of cemented tungsten carbide. This is a common configuration for PDC's used as cutting elements, for example in fixed cutter or rolling cutter earth boring bits when received in a socket of the drill bit, or when fixed to a post in a machine tool for machining.
Another form of PDC element is a unitary PDC element without an integral substrate where a table of polycrystalline diamond is fixed to a tool or wear surface by mechanical means or a bonding process. These PDC elements differ from those above in that diamond particles are present throughout the element. These PDC elements may be held in place mechanically, they may be embedded within a larger PDC element that has a substrate, or, alternately, they may be fabricated with a metallic layer which may be bonded with a brazing or welding process. A plurality of these PDC elements may be made from a single PDC, as shown, for example, in U.S. Pat. Nos. 4,481,016 and 4,525,179 herein incorporated by reference.
PDC elements are most often formed by sintering diamond powder with a suitable binder-catalyzing material in a high-pressure, high-temperature press. One particular method of forming this polycrystalline diamond is disclosed in U.S. Pat. No. 3,141,746 herein incorporated by reference. In one common process for manufacturing PDC elements, diamond powder is applied to the surface of a preformed tungsten carbide substrate incorporating cobalt. The assembly is then subjected to very high temperature and pressure in a press. During this process, cobalt migrates from the substrate into the diamond layer and acts as a binder-catalyzing material, causing the diamond particles to bond to one another with diamond-to-diamond bonding, and also causing the diamond layer to bond to the substrate.
The completed PDC element has at least one matrix of diamond crystals bonded to each other with many interstices containing a binder-catalyzing material metal as described above. The diamond crystals comprise a first continuous matrix of diamond, and the interstices form a second continuous matrix of interstices containing the binder-catalyzing material. In addition, there are necessarily a relatively few areas where the diamond to diamond growth has encapsulated some of the binder-catalyzing material. These “islands” are not part of the continuous interstitial matrix of binder-catalyzing material.
In one common form, the diamond element constitutes 85 % to 95 % by volume and the binder-catalyzing material the other 5 % to 15 %. Such an element may be subject to thermal degradation due to differential thermal expansion between the interstitial cobalt binder-catalyzing material and diamond matrix beginning at temperatures of about 400 degrees C. Upon sufficient expansion the diamond-to-diamond bonding may be ruptured and cracks and chips may occur. Also in polycrystalline diamond, the presence of the binder-catalyzing material in the interstitial regions adhering to the diamond crystals of the diamond matrix leads to another form of thermal degradation. Due to the presence of the binder-catalyzing material, the diamond is caused to graphitize as temperature increases, typically limiting the operation temperature to about 750 degrees C.
Although cobalt is most commonly used as the binder-catalyzing material, any group VIII element, including cobalt, nickel, iron, and alloys thereof, may be employed.
To reduce thermal degradation, so-called “thermally stable” polycrystalline diamond components have been produced as preform PDC elements for cutting and/or wear resistant elements, as disclosed in U.S. Pat. No. 4,224,380 herein incorporated by reference. In one type of thermally stable PDC element the cobalt or other binder-catalyzing material in conventional polycrystalline diamond is leached out from the continuous interstitial matrix after formation. While this may increase the temperature resistance of the diamond to about 1200 degrees C., the leaching process also removes the cemented carbide substrate. In addition, because there is no integral substrate or other bondable surface, there are severe difficulties in mounting such material for use in operation.
The fabrication methods for this “thermally stable” PDC element typically produce relatively low diamond densities, of the order of 80 % or less. This low diamond density enables a thorough leaching process, but the resulting finished part is typically relatively weak in impact strength.
In an alternative form of thermally stable polycrystalline diamond, silicon is used as the catalyzing material. The process for making polycrystalline diamond with a silicon catalyzing material is quite similar to that described above, except that at synthesis temperatures and pressures, most of the silicon is reacted to form silicon carbide, which is not an effective catalyzing material. The thermal resistance is somewhat improved, but thermal degradation still occurs due to some residual silicon remaining, generally uniformly distributed in the interstices of the interstitial matrix. Again, there are mounting problems with this type of PDC element because there is no bondable surface.
More recently, a further type of PDC has become available in which carbonates, such as powdery carbonates of Mg, Ca, Sr, and Ba are used as the binder-catalyzing material when sintering the diamond powder. PDC of this type typically has greater wear-resistance and hardness than the previous types of PDC elements. However, the material is difficult to produce on a commercial scale since much higher pressures are required for sintering than is the case with conventional and thermally stable polycrystalline diamond. One result of this is that the bodies of polycrystalline diamond produced by this method are smaller than conventional polycrystalline diamond elements. Again, thermal degradation may still occur due to the residual binder-catalyzing material remaining in the interstices. Again, because there is no integral substrate or other bondable surface, there are difficulties in mounting this material to a working surface.
Efforts to combine thermally stable PDCs with mounting systems to put their improved temperature stability to use have not been as successful as hoped due to their low impact strength. For example, various ways of mounting multiple PDC elements are shown in U.S. Pat. Nos. 4,726,718; 5,199,832; 5,025,684; 5,238,074; 6,009,963 herein incorporated by reference. Although many of these designs have had commercial success, the designs have not been particularly successful in combining high wear and/or abrasion resistance while maintaining the level of toughness attainable in non-thermally stable PDC.
Other types of diamond or diamond like coatings for surfaces are disclosed in U.S. Pat. Nos. 4,976,324; 5,213,248; 5,337,844; 5,379,853; 5,496,638; 5,523,121; 5,624,068 all herein incorporated by reference for all they disclose. Similar coatings are also disclosed in GB Patent Publication No. 2,268,768, PCT Publication No. 96/34,131, and EPC Publications 500,253; 787,820; 860,515 for highly loaded tool surfaces. In these publications, diamond and/or diamond like coatings are shown applied on surfaces for wear and/or erosion resistance.
In many of the above applications physical vapor deposition (PVD) and/or chemical vapor deposition (CVD) processes are used to apply the diamond or diamond like coating. PVD and CVD diamond coating processes are well known and are described for example in U.S. Pat. Nos. 5,439,492; 4,707,384; 4,645,977; 4,504,519; 4,486,286 all herein incorporated by reference.
PVD and/or CVD processes to coat surfaces with diamond or diamond like coatings may be used, for example, to provide a closely packed set of epitaxially oriented crystals of diamond or other superhard crystals on a surface. Although these materials have very high diamond densities because they are so closely packed, there is no significant amount of diamond to diamond bonding between adjacent crystals, making them quite weak overall, and subject to fracture when high shear loads are applied. The result is that although these coatings have very high diamond densities, they tend to be mechanically weak, causing very poor impact toughness and abrasion resistance when used in highly loaded applications such as with cutting elements, bearing devices, wear elements, and dies.
Some attempts have been made to improve the toughness and wear resistance of these diamond or diamond like coatings by application to a tungsten carbide substrate and subsequently processing in a high-pressure, high-temperature environment as described in U.S. Pat. Nos. 5,264,283; 5,496,638; 5,624,068 herein incorporated by reference for all they contain. Although this type of processing may improve the wear resistance of the diamond layer, the abrupt transition between the high-density diamond layer and the substrate make the diamond layer susceptible to wholesale fracture at the interface at very low strains. This translates to very poor toughness and impact resistance in service.
When PDC elements made with a cobalt or other group VIII metal binder-catalyzing material were used against each other as bearing materials, it was found that the coefficient of friction tended to increase with use. As described in European Patent specification number 617,207, it was found that removal (by use of a hydrochloric acid wipe) of the cobalt-rich tribofilm which tended to build up in service from the surface of the PDC bearing element, tended to mitigate this problem. Apparently, during operation, some of the cobalt from the PDC at the surface migrates to the load area of the bearing, causing increased friction when two PDC elements act against each other as bearings. It is now believed that the source of this cobalt may be a residual by-product of the finishing process of the bearing elements, as the acid wipe remedy cannot effectively remove the cobalt to any significant depth below the surface.
Because the cobalt is removed only from the surface of the PDC, there is no effective change in the temperatures at which thermal degradation occurs in these bearing elements. Therefore the deleterious effects of the binder-catalyzing material remain, and thermal degradation of the diamond layer due to the presence of the catalyzing material still occurs.
There have also been attempts in this art to use traditional leaching methods to solve the problem that describes to make them more temperature resistant. These traditional leaching methods have involved the leaching of the entire diamond table or a majority of it.
The traditional leaching method involves the use of highly concentrated acids, such as nitric, sulfuric and/or hydrofluoric, raised to near the boiling points of such acids. In such process, the PDC cutters are placed in a bath of one of these acids diamond side down. These attempts in the prior art treat the entire diamond surface or the biggest part of it. These attempts are shown in U.S. Pat. Nos. 6,739,214, 6,592,985, 6,749,033, 6,797,326, 6,562,462, 6,585,064 and 6,589,640. The same technology, having the same shortcomings, is found in U.S. Pat. No. 4,224,380 to Bovenkirk, et al., assigned to General Electric, and Published Japanese Patent Application Number 85-91691, assigned to Sumitomo. These patents typically designate specific leaching depths and all these patents address treating the entire compact, or are based on depth from the face of the diamond surface. Thus, when the cutters are exposed to the heated acid, the acid itself will remove the cobalt in the interstices of the matrix which is proposed to make them less likely to fail due to high temperatures. The problem with this approach, is that when the cobalt or other metal is removed from the interstices of the matrix, the material is not as strong mechanically and can cause the cutters to break off. The only reason the cobalt is formed in the matrix in the first place is to make them more mechanically stable but when that portion of the cobalt or other metal is removed, the cutters become less impact resistant and thus less mechanically stable. When drilling a hole with a PDC bit having PDC cutters, such as used in drilling in oil and gas well, only the repeat downward oriented edge of any PDC cutter is doing the cutting work. In general, maintaining the integrity of this sharp drilling edge is the focus of the leaching treatment. Because the cutters are round, typically, and their installation as to orientation is uncertain, those in this art have leached the entire PDC layer. Yet, when this drilling edge is worn down by abrasive formations, those full face leaching cutters sometimes fail nearly as rapidly as the non-leached cutters due to heat generation on the large wear flat of the PDC cutter. These prior cutters are also more fragile with respect to impact than the non-leached cutters, all as discussed hereinabove.
Each of the U.S. Pat. Nos. 6,739,214; 6,592,985; 6,749,033; 6,797,326; 6,562,462; 6,585,064; 6,589,640; 4,224,380 (Bovenkirk, et al) and Published Japanese Application Number 95-91691 (Sumitomo) are incorporated herein by reference for what they disclose. However, each of these references disclose leaching of the metallic phase, typically cobalt, commencing with the entire face of the diamond surface, coupled with a continued leaching of the cobalt over a depth range of 100 or 200 microns from the face up through and sometimes including the entirety of the diamond compact.
The depth of the acid leaching process is a function of many factors. These factors include the following items, and for any given acid leaching process, some or all of these elements may or many not be involved:                The nature of the metallic phase; this will often involve cobalt but other known metallic components can be, and are used in the manufacturing process of constructing polycrystalline diamond compact cutters for use in drill bits;        The extent to which the diamond crystals themselves are “finer” in size; some PDC cutter manufacturers use “fine” diamond crystals, for example, US Synthetic Corporation and E6, a DeBeers Company. Each use fine diamond crystals in making PDC cutters. As a general rule, the finer crystals have smaller interstitial spaces between the crystals, resulting in a smaller amount of cobalt to be leached out.        The chemical composition of the acid used in the leaching process; the most common acids used in this process are hydrochloric acid, nitric acid, hydrofluoric acid, sulfuric acid and various mixes thereof; some of these acids are more aggressive than others in leaching a given metallic phase, and the volumetric ratio of one acid to one or the other acids also effects the aggression of the acids used in the leaching process;        The temperature of the acid used in the leaching process; as a general rule, the acids used are more aggressive when used at or near their respective boiling points;        The time of exposure of the metallic phase to the leaching acid; everything else being equal, the elapsed time of exposure is the most important factor in determining the depth of the leaching process.        
For example, in the '380 patent to (Bovenkirk) et al, selected samples were leached in a mixture of hydrofluoric acid and nitric acid taking between eight and twelve days to entirely remove the metallic phase.
With a second set of samples, the hydrofluoric acid-nitric acid was alternated with aqua regia (hydrochloric acid-nitric acid) for a period of three to six days, removing entirely the metallic phase. Thus the other factors above set forth determine the rate at which the depth of leaching occurs and the depth of leaching is only a function of time. Assuming the rate of leaching is determined by specifying the acid mix, the operating temperature of the acid mix, the diamond particle size, the given metallic phase, e.g. cobalt, the depth of leaching is determined to be X microns of depth per hour. In Y hours the depth of leaching is merely XY microns.
From a practical standpoint, in truly abrasive rock formations, full face leached cutters also wear and the wear flat is usually large enough that the PDC cannot be rotated for repair. This results in the cutter being essentially useless even though it has an expensive chemical treatment across the entire diamond table. This results in portions of each cutter that are never used due to large wear flat development, a development which often extends into the cutter pocket in highly abrasive formations. The present invention contemplates that only a selected portion or portions of the PDC cutter are leached.